i) Technical Field of the Invention
This invention relates to a CATV amplifier provided on a transmission line from the center equipment of a bi-directional CATV system to terminal devices, respectively amplifying upward signals and downward signals bi-directionally flowing through the transmission line.
ii) Description of the Related Art
Conventional CATV amplifiers are provided on a trunk connected to center equipment and branch lines, branching from the trunk, in a bi-directional CATV system. They amplify bi-directional signals flowing through the transmission lines. Such CATV amplifiers (specifically, CATV trunk amplifiers, CATV trunk bridger amplifiers, CATV extender amplifiers, and CATV bridger amplifiers provided on the branch lines branching from the trunk) comprise a downward amplifying circuit for amplifying a downward signal transmitted from the center equipment to the terminal devices, and an upward amplifying circuit for amplifying an upward signal transmitted from the terminal devices to the center equipment.
In recent years, not only a conventional frequency band (for example, ranging from 5 to 42 MHz) lower than the frequency band of the downward signal (for example, ranging from 54 to 722 MHz) but also a frequency band (for example, ranging from 770 to 890 MHz) higher than the frequency band of the downward signal, are set for the frequency bands of the upward signals in order to increase the amount of information transmitted from the terminal devices to the center equipment. Accordingly, a bi-directional CATV system has been proposed using these two types of frequency bands to transmit a higher volume of upward signals.
A CATV amplifier used in such a system is required to amplify the upward signals in different frequency bands. Therefore, as disclosed in the Unexamined Japanese Patent Publication No. 2-127824 for example, an upward L signal path, for passing a low-frequency upward signal (hereinafter, referred to as an upward L signal), and an upward H and downward signal path for passing a high-frequency upward signal (hereinafter, referred to as an upward H signal) and a downward signal, are formed by the use of a pair of directional wave filters connected to the input and output terminals for transmission signals connected to the transmission lines on the center equipment side and on the terminal device side, respectively. Then, an upward H signal path for passing the upward H signal and a downward signal path for passing the downward signal are formed by the use of a pair of directional wave filters arranged at both ends of the upward H and downward signal path. Lastly, amplifying circuits for amplifying each of the signals (the upward L signal, the upward H signal, and the downward signal) are provided on these paths (an upward L signal path, an upward H signal path, and a downward signal path).
However, in the conventional CATV amplifier constituted as above, the upward H signal, of which the transmission losses in the transmission line increase the most of these three types of transmission signals flowing through the transmission line of the bi-directional CATV system, has to pass through two directional wave filters before being input into the amplifying circuit (hereinafter, referred to as an upward H amplifying circuit). In addition, the upward H signal, after being amplified by the upward H amplifying circuit, also has to pass through two directional wave filters. As a result, there is a problem in that the transmission losses of the upward H signal in the conventional CATV amplifier increase twice as large as the transmission losses of the upward L signal, and the CN ratio (carrier-to-noise ratio) of the upward H signal is reduced.
Furthermore, it is necessary to increase the gain of the upward H amplifying circuit in order to compensate for the transmission losses of the upward H signal, and to secure the CN ratio. For that purpose, the power supply to the upward H amplifying circuit has to be increased. Thus, there arises another problem in that the power consumption of the entire CATV amplifier is also increased.
Specifically, since the transmission losses of the upward H signal are larger than the transmission losses of the other transmission signals (the downward signal and upward L signal), it is necessary to set the gain of the upward H amplifying circuit to be larger than the gain of the other amplifying circuits (the downward amplifying circuit and upward L amplifying circuit) used for amplifying the other transmission signals. In the above-described conventional CATV amplifier, the transmission losses generated when the upward H signal passes through a passing path in the amplifier become larger than the transmission losses of the upward L signal due to the two directional wave filters. Therefore, the gain of the upward H amplifying circuit has to be made larger than the gain of the other amplifying circuits.
In order to accomplish the above task, the gain of the transistors used for amplifying the signals, constituting the upward H amplifying circuit, has to be made larger than the gain of the transistors used for the other amplifying circuits. Alternatively, the connection number of the transistors has to be set larger than the connection number of the other amplifying circuits. Then, the energy supplied to the upward H amplifying circuit also has to be set larger than the energy of the other amplifying circuits. Accordingly, the power consumption of the entire CATV amplifier is also caused to increase.
Furthermore, as the power consumption of the CATV amplifier is increased, distortion due to the hum modulation generated in the CATV amplifier grows larger. Therefore, in a general bi-directional CATV system supplying power from a power unit to a plurality of CATV amplifiers via the transmission lines, there is another problem in that the distortion due to the hum modulation cannot be ignored.
Specifically, in the bi-directional CATV system a power unit, transforming the commercial power of 50 Hz or 60 Hz to generate alternating current power signals of tens of V, is provided at one per several CATV amplifiers. The alternating current power signals generated by this power unit are supplied to each CATV amplifier via the transmission line. The distortion due to the hum modulation grows larger as the energy supplied to the internal amplifying circuit of each CATV amplifier is increased, and also grows larger in the entire system as the cascading number of CATV amplifiers is increased. Therefore, when the power consumption per CATV amplifier increases as above, the distortion due to the hum modulation generated in the entire system cannot be ignored.